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Why use a RAT when engines can windmill?
When a dual engine flame-out is encountered, and the engines themselves are not damaged such as in the case of Air Transat Flight 236, why is the RAT deployed for emergency power instead of using the engine generators?
I imagine that the engines themselves will still be spinning sufficiently quickly in the wind to generate enough electrical power. Of course, I'm not saying that a RAT should be removed, because in a different scenario, both engines could simply explode and their generators would be useless. But I'm wondering why, in the case of fuel exhaustion, the engine generators are not used at all, and we rely only on the RAT / batteries. Is it because connecting a generator to windmilling engines would slow them down too much and causes unnecessary drag during the glide? Or is it because the engine generators are optimized for much higher rotational speeds?

The engine's generator is connected to the engine core, and the core can never windmill to the needed speed. It'll be a different story if (one of) the generators are mounted on the fan. If so then you could indeed get rid of the RAT because the fan windmills much faster.

Isn't the fan directly connected to the core via the LP shaft? In that case, both the fan and the LP core rotate at the same angular speed (RPM), and could produce the same torque.

@natanijelvasic The fan is usually (not on RR Trent engines) connected to the LPC and LPT (low pressure compressor and turbine), but the generator is connected via the accessory gearbox to the HPC and HPT (high pressure compressor and turbine).

@Bianfable I find that hard to believe, where is the fan connected to then? All turbofan engines I have modeled in the past 25 years are connected to the LPT...

@0scar RR Trent engines have a 3 spool design: the fan is connected to the LPT, but not the LPC (which is powered by the IPT). Anyway, the accessory gearbox is still connected to HPC and HPT (the N3 stage for RR Trent).

It seems that you have actually answered yourself

@Bianfable I believe the LPC you refer to is called the IPC in RR engines. The fan is connected to the LPT. The IPC to the IPT and the HPC to the HPT.

@0scar You're right, somehow I thought they still call it low and high pressure compressor, but you're correct: I meant the IPC.

There are a few things to consider:
In an Airbus (such was the case with Air Transat Flight 236) when both engines are lost the RAT will automatically deploy (see the answer to this question) so in the case of fuel exhaustion or any dual engine failure its happening regardless of pilot action.
That would be bad aircraft design. You shouldn't design an aircraft to rely on anything failing in a particular way. When designing a backup power system, and subsequent checklists for its use, the assumption is that there has been a critical failure of the whole component. What if the engines shear off? What if its a dual fire situation? Dual fan blade impact and they are not spinning? The RAT covers all these cases.
You could perhaps add this as a troubleshooting step somewhere in an additional flight manual.

You're completely right - the RAT should deploy regardless, and assume that the engines have failed completely. So I guess the reason why "scavenging" power from windmilling engines is a bad idea, is because a transition to a complete engine failure might put your plane in an unrecoverable configuration. For example, the pilot might extend flaps using the "scavenged" engine power, the engines might then fail completely leaving only the RAT, and the flaps can no longer be raised.

One should never plan on having an emergency backup of a critical component reliant on the successful operation of one of the systems the emergency backup supports.  You want completely independent solutions that do not have any dependence on each other.

Because why would you want to do that?
As you say in the question, the plane already needs a RAT, and the RAT and batteries are already sufficient to get you down safely (because you can't rely on anything else working).  You don't need more electrical power.  How does it help?
Because why would you want the crew to do that?
We can all agree that losing both engines qualifies as an emergency.  In an emergency, the priority is flying and navigating the plane safely.  Everything else is secondary, and the fewer extra tasks you have there, the better.
Humans have a limited processing capacity, and a number of emergencies/malfunctions have been converted into accidents purely by overloading the crew.  You don't want your crew wasting time trying to figure out exactly why or how the engines failed, or assuming incorrectly that they can rely on the engines and then losing electrical power entirely, or misreading a complex fault-finding procedure and losing electrical power as a result, or trying to follow a complex fault-finding procedure and forgetting to fly the plane whilst doing that.  That's all nonsense, and worse than that, it's entirely unnecessary nonsense which has literally killed people.  I'm assuming your intention here isn't some sadistic "let's maximise casualties" mission? ;)  If you want the crew and pax to have better odds, give the crew one solution which works, and they just do it and move on.

Yes that makes sense. As I've commented above - we don't want the pilots to rely on partially failed components, only to discover later that they can't anymore, resulting in an unrecoverable configuration.

I think @natan's "how does it help" idea is that it might improve the glide ratio, something that matters in some dual-engine-out situations.  The engines are going to be in the airstream anyway, so if increasing their drag (by the braking action of a generator) can extract sufficient power more efficiently than having the RAT out in the airstream, you win a slight reduction in drag vs. the RAT.  Of course, even if it would work at all (generator on a part that windmills fast enough), it's probably not much of a drag difference, and as you say, crew task saturation is critically important.

The engine generators are designed to run at the engine idle speeds, which are higher than the speeds that the engines would be windmilling. Working with a larger range of engine speeds would add design challenges.
Another issue is that the generator is usually attached to the N2 high pressure spool while the fan is attached to the N1 low pressure spool. Windmilling would primarily be rotating the fan and not the spool that the generator is actually attached to.
It would certainly be theoretically possible to have an engine generator designed to run while the engine is windmilling. But as others have pointed out, this wouldn't help in all situations where the RAT is needed, and the RAT is more efficient and reliable being designed specifically for this purpose.

Good point about the high pressure spool. Now I'm wondering what typical N1 and N2 values are during glide.

Most jets with a RAT that provide emergency AC power to the aircraft have constant speed drive generators.  These generators spin as a certain RPM (say 12,000 RPM) to provide the requisite 400 Hz AC power for the electrical system.  If the generators spin at a different speed the hertz will fluctuate and, in an AC powered airplane, that would cause additional issues.
An engine that is windmilling cannot create enough rotational torque to spin the AC generator at the required RPM for power.

I haven't done the maths, but it does seem a bit odd that a small RAT can produce the required torque for the constant speed drive, and the windmilling engines can't. Do you think it's because the engine's constant speed drive is only designed to operate at a torque and RPM range under normal, powered operation?

No. The traditional setup uses the RAT to generate hydraulic power which feeds a separate hydraulic-powered generator to create electrical power, preventing the need for constant RAT speed. This includes the A300-A340, 747-8, 757, 767. The 777's RAT generator only supplies transformer/rectifiers (switching power supplies) which supplies DC. (The 777's backup engine generators feed a cycloconverter, so they aren't constant-speed either.) The A350, A380, and 787 have variable frequency AC systems, so like their engine-driven generators, their RAT-integrated generators are not speed regulated.


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What's the purpose of the cable connected on the top of the fin?
A321neo has on the top of the fin, backwards, a long tube, which looks like the pitot tube on the Boeing 707, but on the opposite way, and connected on it, a cable (I think because it is a test flight).
What is the purpose of each one?
Picture's credits: www.airbus.com/presscentre

It is a Trailing Static or Trailing Cone used in test aircraft to measure free stream values. From Airdata Measurement and Calibration:

By trailing a long tube behind the airplane, a nearly free-stream static pressure measurement can be taken. A perforated cone at the end of the tube acts as a drag device to keep the tube stable.

Image from NASA Airdata measurement and calibration
It is used in test aircraft. The image below shows the trailing cone attached to the 787 dreamliner (pictured at Paris airshow).

Image from flyingsquadron.com
This image shows the trailing cone assembly used in 787 dreamliner.

Image from airpigz.com
The device is specified by FAA and JAA as one of several methods that can be used to perform flight calibrations. According to FAA Advisory Circular 91-85:

Where precision flight calibrations are used to quantify or verify altimetry system performance they may be accomplished by any of the ollowing methods. ...
• Precision tracking radar in conjunction with pressure calibration of
  atmosphere at test altitude.
• Trailing cone.
• Pacer aircraft.
• Any other method acceptable to the FAA or approving authority.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

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How safe is the single engine taxi out for twin engine aircraft?
Fuel conservation programs are leading to single engine taxi out, asking the crew to start the other engine near or on the position 2, where the ground crew cannot monitor the engine's status outside the aircraft.
How safe is this procedure? One example is the jet pipe fire during the start up.

Are you assuming that mechanics always monitor engines during startup closer to the gate? Even closer to the runway, other aircraft or ATC should still be paying attention. They tend to notice things like engines on fire.

Are they responsible for that? I mean, from a distance, it could seem to be, or even to late for an instant action. couldn't it?

Well ATC is responsible for keeping everyone moving safely, and have been faulted when not paying enough attention, so I would say they are somewhat responsible in this role.

I agree partially. For sure they are responsible for this and many other actions, and they would inform the crew a little bit late, while the mechanicas are responsible for this, during the start up procedures.

Mechanics are not responsible for startups, in fact they are very rarely present for engine starts, so regardless of it being at the gate, or the ramp, or the taxiway, or the runway, the responsibility lies with the pilots in the aircraft and people observing on the ground to report things out of the ordinary.

I disagree with your post, the mechanics (ramp personnel) assist the pilot while they are pushing back, and starting the engines as well.

FWIW, before my retirement in 1999, mechanics were rarely present for engine starts, and to refer to a mechanic as a ramp person would have been highly insulting to a mechanic. Ramp personnel, at least back then, were not licensed, and were paid far less than mechanics, who, of course, had to have A&E licenses. Actually, as I remember, more often than not, we referred to ramp personnel as ramp rats.

As I am neither a pilot nor airport agent, I am allowed to make mistakes, and to me, and most of the passengers, the ground crew who is connected to the aircraft by the radio system would be mechanics, however, I'd never call them as "rats", to us they are as essential as pilots.

they aren't essential at all. A pilot (or a properly trained mechanic) can taxi a plane without a ramp rat, but a ramp rat can't taxi a plane without a pilot. those orange flashlights the ramp rats use don't actually move the aircraft :)

I am not aware of mechanics watching civil aircraft start up. They usually have far better things to do. Aircrew are responsible for the aircraft at this point.
The question on a twin is about directional control not safety. How easy is it to taxi on one engine without losing control or stressing the aircraft or tyres in any way? I think this probably depends on the aircraft, some will be easier than others. Also if its a piston twin, you don't want to wait for the other engine to warm up when you are ready for departure. Some four engined aircraft might taxi on two engines - one either side but I've never heard of twins doing this routinely.

Fuel conservation program requires, for example, LATAM Airlines, the A320/21s perform the taxi in and out, with only the engine number 2 running.

Interesting. I was thinking more of light twins with piston engines.

I see saab 2000s do this quite often

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Is it possible to survive a free fall from an airliner at cruise altitude?
If an airliner were to breakup in flight, and a passenger were ejected from the cabin, would it be possible to survive the fall?
Assume that the aircraft broke up at a cruise altitude 30,000-35,000 feet. If it improved the odds, suppose the person were wearing a heavy winter jacket and a backpack with additional clothing in a plastic bag.
Is is possible the person could survive the fall? Are there steps the person could take that might increase the chances of survival?

You may not survive.

Extraordinarily good luck.

Related question about parachute failure when skydiving

It's not the fall that will kill you, it is the abrupt end of the fall that does you in.

"JAT stewardess Vesna Vulović survived a fall of 10,000 metres (33,000 ft) on January 26, 1972 when she was aboard JAT Flight 367. The plane was brought down by explosives over Srbská Kamenice in the former Czechoslovakia (now the Czech Republic)." (Source).

If you want to discuss the situation from Vesna Vulcovic (see answer from Dave) try skeptics.se . physics.se might give you answers about the impact speed on ground and the possibly damage to the body.

https://en.wikipedia.org/wiki/Nicholas_Alkemade Related: survived a fall from a bomber at 18,000ft, suffering a sprained leg. Hitting trees down the side of a hill, followed by a snow bank, was what saved his life. Possible, but unlikely.

Chances are you would not survive but lets explore this a bit more in depth for the sake of education.
This article from popular mechanics explores the very issue. There is some research that has gone into the topic of hitting the ground but the research is rarely about cases from the height you are asking about (~30,000ft). Some relevant info
First off at that altitude you really can't breathe that effectively so hypoxia will be your first concern followed closely by extreme cold.
Hitting water:

Studies of bridge-jump survivors indicate that a feet-first,
  knife-like entry (aka "the pencil") best optimizes your odds of
  resurfacing.

Hitting the ground:

The question of how to achieve ground contact remains, regrettably,
  given your predicament, a subject of debate. A 1942 study in the
  journal War Medicine noted "distribution and compensation of pressure
  play large parts in the defeat of injury." Recommendation: wide-body
  impact. But a 1963 report by the Federal Aviation Agency argued that
  shifting into the classic sky diver's landing stance—feet together,
  heels up, flexed knees and hips—best increases survivability.

Your best bet for surviving the airplane break up is if you are stuck in a situation similar to that of Vesna Vulović who did survive the situation you describe. Unfortunately there is some debate as to what actually happened so take the info at face value.

“Speed has never killed anyone. Suddenly becoming stationary, that's
  what gets you.”  ― Jeremy Clarkson

It's worth noting that if you pass out from hypoxia or hypothermia (both pretty likely given the low partial-pressure of oxygen and extreme cold at typical jet cruising altitudes) you may not wake up in time to adopt any landing position when you hit whatever's below you. On the "bright" side being unconscious combined with the low odds of surviving the landing impact means you probably won't care how you land anyway.

Perhaps the best bet: hit a snowbank & get rescued quickly: {Early 20th century} "Literature discussing the use of parachute troops is published. The concept is considered amusing in most countries but is taken seriously in Germany and in Russia. The Russians practice dropping troops from airplanes in deep snow without parachutes. There are few injuries from shock, but too many fatalities from suffocation. The concept is abandoned."

I have nominated the question for re-opening, just for this excellent answer

@voretaq7: It will also depend the duration of exposure of extreme temperature, as the subject will lose altitude drastically, as he would be travelling faster than the terminal velocity..

Another example: https://en.wikipedia.org/wiki/Nicholas_Alkemade

According to Wikipedia's article on terminal velocity,

the terminal velocity of a skydiver in a belly-to-earth (i.e., face down) free-fall position is about 195 km/h (122 mph or 54 m/s).

Using this online physics calculator with

Acceleration: 98.1 m/s^2 (10G)
  Final velocity: 0
  Initial velocity: 54 m/s

I get a deceleration distance of -9.42 meters, or about 31 feet.
If you can somehow slow down over that distance (perhaps by hitting a stand of trees and some snow, preferably on a hill side) you might make it.
Note: the 10G deceleration is completely arbitrary on my part - I assume it's a reasonably survivable impact depending on what and how you hit.

10 G seems pretty restrictive -- two or three times that is probably survivable for a brief period. Hurt like heck & sustain injuries, but fighter pilots routinely pull 9 Gs sustained without injury. The G-suit aids in retaining consciousness, but Red Bull pilots pull Gs like that without a suit & do okay.


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What exactly are "suitcase handles" in a cockpit?
I was watching Seconds From Disaster about Alaska Airlines 261 (the one with the horizontal stabilizer issue) and at one point the copilot tells the pilot to "try the suitcase handles". Is that pilot slang for something or did I maybe hear it wrong? (Seconds From Disaster uses actual cockpit recordings for the recreations, so I don't think it's made up or anything.)

The issue was with the horizontal stabiliser, not the vertical.

Edited by changing “vertical” to “horizontal”.

From the NTSB report

"Suitcase handles" is a colloquial term for the longitudinal trim handles located on the center control
  pedestal. "Pickle switches" is a colloquial term for the trim switches located on the outboard side of each of
  the control wheels.

The MD-80 cockpit is shown at this page, with the trim handles on the left side of the console (item 9).

Under normal operation, the pilot will use the switches on the control column to change the trim.  If there's a problem with the thumb switches, the handles on the console are available.

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Where can over age 65 airline pilots fly?
Some countries have no maximum age for airline pilots while many other countries specify age 60 or 65. (Austria, Russia, Canada, Australia, etc., have no maximum age)
ICAO rules currently specify a max age of 65 which allows ICAO member states to fly into countries where the max age is only 60.
Does this rule apply to only landing and taking off, or also to in transit overflights?
For example, could a flight depart Canada, overfly American airspace, and fly to Australia with crew members older than 65?
If not, what about a flight from two bordering countries like Canada to Russia?

Part of this is covered in this question/answer here on the site.

Where exactly is the Canada-Russia border?

@DJClayworth I assume OP is thinking of a polar route.

Yes, Russian airspace borders Canadian airspace.

Pilot qualification- including age limit - is according to the law of the country where the aircraft is registered. What a pilot must abide by when flying overseas is the local operating/air traffic rules.

Sorry, that is not really correct.  ICAO rules allow the pilots of member states to fly into countries without meeting the age requirements of local countries. France for example has a max age of 60 yet airlines from all over the world routinely fly into France with pilots over age 60.


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Why aren't there direct flights connecting Honolulu, Hawaii and London, UK?
London is by far my favorite city, and many people in Honolulu apparently feel the same. An old poll (5-6 years ago) said that 50% of polled people would go to London on that flight if it existed. The airlines are quite definitely aware of this, as Hawaiian Airlines responded a year later by saying they would make a direct flight. But it never happened, and no one has heard anything from them on the issue since. (This was more than four years ago.) I have considered the following:

Capabilities of planes (More than capable of making the flight)
Popularity of flight (The poll)
Flight restrictions (None)

After my pondering, I have concluded that there should be no problems connecting the two popular destinations. So why are there no flights? They would mean certain profit for the first airline to offer the journey. (They could charge whatever they wanted to.)

"They would mean certain profit for the first airline to offer the journey." What makes you think that you know better than the people who run actual airlines? "(They could charge whatever they wanted to.)" No they couldn't: it's already possible to fly from Honolulu to London, just not direct; people will only pay so much extra for a direct flight versus one with connections.

I would like to go to London as well. But I always find an excuse to stay lazy. If it's not "there are no direct flights", I'll make up another one.

"They could charge whatever they wanted to" -- if the poll had asked, "would you pay $100,000 to take this flight" they would have got less than 50% of "yes" answers. So they can only charge what the market will bear, and you don't report what those polled said they'd be willing to pay for this flight (let alone what they'd really pay)! I also wonder who they polled: if n% of Hawaiians actually have visited London, then is it really true that (50-n)% of Hawaiians are prevented from visiting London solely because they currently have to change flights?

British Airways looked at this but there are many other destinations in North America they would rather serve. (For instance you may wish to keep an eye on MSY's news website today.) The fact is that there is not enough business class travel between London and Hawaii to justify the high cost of running a sixteen hour non-stop; and you have to remember that BA want to use their aircraft in the most profitable way possible, not just run routes because they might be marginally profitable. For those of us going to HNL regularly from London, it is easy enough to change planes in LAX, PHX or DFW.

@Calchas Yeah, I know BA has been looking at several different U.S. cities for possible expansion, especially with their 787s. BNA has been hoping to attract them for a while, but hasn't won out so far.

Simply, because no airline has determined the route makes economic sense.
A poll doesn't necessarily translate into a ready market of passengers. Sure, lots of people might check a box on a survey saying they'd like to see such a thing, but does that really translate to thousands of people actually a significant sum of money for the service? Would enough of them travel frequently enough at a high-enough price point to make the service worthwhile? There's no guarantee of "certain profit," and considerable money could be lost investing in such a route if it turns out not to be a success. A few reasons why it might not be a slam dunk:

Some passengers already travel between London and Hawaii, through connecting flights. A non-stop flight might cause more people to fly this route, but it would also cannibalize existing business from the connecting flights, meaning you'd need to get further customers to make up the difference.
LHR-HNL is about 7,200 miles. As you note, this should be within the capabilities of long-range aircraft, but we'd still be talking about a flight on the order of 16 hours. This is quite the distance. Would enough passengers want to travel this long between the cities? To put it another way, are passengers willing to travel that long also willing to endure a bit longer of a journey by making it a connecting flight?
The airline operating the route needs aircraft capable of such long flights. If they don't already operate a suitable aircraft, they need to acquire some and train pilots and mechanics and ground staff, an operation that takes years and hundreds of millions of dollars.
Hawaii is generally considered a leisure destination, and it's generally a fairly expensive one at that. Londoners have access to lots of rather inexpensive flights on low-cost-carriers to beach resorts within at most a few hours flying time. Many are in low cost of living countries where accommodation is fairly cheap. Tenerife, Greece, Southern Spain, Mallorca, Cyprus, and a number of Eastern European destinations are all rather accessible, and operators frequently offer packaged trips. Such destinations may not be exactly the same as all the splendor of Hawaii, but they're much cheaper and don't require 16 hour flights to get there.
Will the route be popular with business travelers? Keeping such a service profitable is far easier if you have a regular stream of business travelers between the two cities, especially if they're willing to pay for premium cabins and last-minute bookings. Ultra-longhaul flights without a large supply of business travelers just don't work.
Landing slots at Heathrow, the most logical place for such a flight due to its connections, are in very short supply and thus quite valuable. This further increases costs.
Ultra-longhaul flights work best when they're connected with an extensive route network, so they can receive feeder passengers from and carry passengers to other connecting flights. That's why they generally serve major airline hubs. Honolulu, because of its location, isn't an ideal high-density hub. Its main links are to other Hawaiian and Pacific islands, along with the US West Coast and East Asia, generally to cities that are already served by direct flights to London. As such, adding such a flight would not be that useful in helping an airline build its network. In addition, if Hawaiian were to operate the route, it would need to develop partnerships with European airlines to build a route network there (so as to facilitate booking, of say, HNL-LHR-CDG), a process that could involve lengthy and expensive negotiations.
Is there significant air cargo demand? A very profitable cargo route may help make up for lower passenger demand, but is there a lot of demand to ship cargo between the two cities? The amount of cargo payload you can take also depends on the aircraft you choose. Some aircraft do have the range to make the flight, but would have payload restrictions, which reduces profitability. Larger aircraft may have more capacity, but may be harder to fill.

Also, according to this forum post, a service was operated in the early 80s (with a fuel stop), and it was rather unpopular and thus unprofitable.
All that said, ultimately, all we can do is speculate. Airlines study potential routes extensively, but they don't release their internal numbers. If one thought it made economic sense, they would introduce the new service.

@SimonRichter:  It's also possible that the nearby destinations (Tenerife, Greece, etc.) will become slightly less convenient after Brexit, causing more UKers to take a fresh look at Hawaii.  But I agree with your general point:  the economic outlook is uncertain.

@MichaelSeifert People's ideas of what is and is not convenient would have to change a heck of a lot for them to want to take an umpteen-hour flight to Hawaii instead of a 4-hour flight to Tenerife or Greece.

You might also point out the fuel usage and how few passengers you could carry with all that fuel. See "fuel tanker in the air" comment here.

You're assuming that most of the demand will come from the London end, which contradicts what the OP has posited (the demand mainly being driven from the Honolulu end)

@SimonRichter I highly doubt that Brexit will significantly change the demand of Hawaiians to travel to London, since the U.S. isn't part of the EU anyway. Convenience for Americans traveling to the U.K. (especially on direct flights) is almost totally unaffected by Brexit. If anything, the slight fall of the pound relative to the dollar would increase said demand. However, it could decrease demand for Londoners going to Hawaii, especially on a U.S.-based airline.

@UnrecognizedFallingObject I'm assuming demand needs to be somewhat balanced, or you'd need to get even more Hawaiians to fly to make up for the difference. Even so, the real point, as reirab notes, is that it would be primarily an ultra-longhaul leisure route. My gut feeling is that there isn't a lot of business demand from either end. Whichever end the demand comes from, if you aren't selling a whole bunch of last-minute Y and C fares to business travelers, it's hard to see how it makes financial sense, especially with so many one-stop connection options.

There are direct flights London to Barbados non-stop, so there are routes from London to a non-hub "holiday" destination further afield than Cyprus (well, it's a hub for the southern caribbean I suppose!). But of course you're right that competition from much-shorter flights to the Med has to be considered, and the historical ties between the UK and Barbados are (I think) rather stronger than with Hawaii. And, although flights to the Caribbean provide a precedent for long-haul fly-and-flop, they also represent even more competition for Hawaii.

Going to the Caribbean is cheaper faster, more convenient if you want to go to a tropical island from London.

@SteveJessop Barbados has a booming financial industry because of its 1% corporation tax on off-shores and favourable tax treaty arrangements with the US and UK. I would not class it as a pure bucket-and-spade route; there are a few major corporates who are buying a lot of the F and J inventory on each flight. But I don't really imagine any company willing to step forward and underwrite LHR-HNL.

@SteveJessop Yeah, there are several direct flights from Europe to Saint Martin, too, but that's only a bit over half the distance of Europe to Hawaii.

Hawaiian Airlines
Hawaiian Airlines simply doesn't have any aircraft that can operate the flight.
For now, Hawaiian Airlines does not consider its A330-200s to have sufficient range to reach London with the desired payloads and it's not yet sure whether its upcoming A330neos will be able to, either.

Flights to London are a goal but may be at the limit of what’s attainable after the carrier swapped its order for Airbus Group SE A350-800 jets to the shorter-range A330neo in 2014, Hawaiian Holdings Inc. CEO Mark Dunkerley said in an interview Wednesday.
“We certainly hope the answer is yes but we don’t have all the information we need,” he said. “It’s going to depend on what our final seating configuration is and therefore the weight of the Neo and its exact performance and statistics when it’s actually built.”

Source: Recent Bloomberg article citing Hawaiian's CEO
Note from the above quote that Hawaiian has replaced the A350 order mentioned in another answer with an order for A330neos instead, so it currently has no A350s on order.
One interesting note from that same interview is that Hawaiian is also looking at possibly acquiring some of the cheap A380s currently floating around on the market. However, it's considering these for its current high-demand markets such as Los Angeles, Las Vegas, and Tokyo, not expansion to Europe.

Dunkerley, 52, said the airline is looking seriously at whether the Airbus A380 might have a role to play within its network, especially on routes such as those from Honolulu to Los Angeles, Tokyo and Las Vegas, which it serves with smaller wide-body planes six, three and two times daily respectively.
The argument for using the superjumbo, which might be available on attractive terms either from Airbus or, soon, in the second-hand market, isn’t conclusive and Hawaiian needs convincing of the business case, Dunkerley said.

In General
The main problem overall with direct flights between Europe and Hawaii (on any airline) is what Zach mentioned in his answer: It's an ultra-long-haul leisure route.
It's very hard to make the numbers work for such a route, as airlines typically rely on selling expensive close-in-booking and premium-cabin tickets to business travelers as well as relying on them to keep load factors up in order to turn a profit for their ultra-long-haul routes. This route would also be competing with several existing single-stop routes with connections in New York, Chicago, Atlanta, Denver, Los Angeles, Seattle, etc. These routes are already economically feasible due to the much higher demand for traffic to London from these connecting points (and other cities feeding to them) than from Hawaii. Similarly, demand from the U.S. mainland to Hawaii is much higher than from Europe to Hawaii, so those existing flights remain viable.
With that said, there are recent rumors regarding the Swiss leisure-route-focused airline Edelweiss possibly starting a route to Hawaii from Zurich soon using A340-300s, but this is presently still unconfirmed. The economics of this seem pretty sketchy, though, and the "as early as December 2016" start date mentioned seems even more sketchy.


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Is a horizontal stabilizer necessary on an aircraft with a tandem wing?
Let's say that there is an aircraft with more than one main wing and each of these wings have elevators. Would it be necessary to have a horizontal stabilizer? The figure below shows an example of what I'm thinking.

No. This configuration is called a tandem wing. For stability, put the rear wing at a smaller incidence and shift the cg forward accordingly. Bummer that this has been closed - I think it's a good question if you take the main wings and stabilizer restrictions out.

@Simon additional wing Look at the pictures of this aircraft. Notice that it has a main wing with 4 engines with a horizontal stabilizer attached to the airplane, but then look at the additional wing on the front of the aircraft and that's what I am asking about.

@Federico Look at the post I just gave simon and maybe that will help a little bit.^

take a decision, you cannot both include and not include the main wings.

In your post, you say each of these wings have elevators, then in the examples you give, none of them has elevators on the wings (which btw, cannot work, the elevators must have a long lever to the CoG).  They also have horizontal stabilisers.  Is your question simply what are the small additional wings at the front?  If so, you are asking about canards.

@Simon Look at google doc.

@Ethan I've added the image. But it still looks like a configuration having a main wing, a canard and a horizontal stabilizer (at-least the spans appear different) . Also, if you already have so many elevators (never mind their location), why do you need another stabilizer and where do you put it?

@aeroalias I'm asking if a stabilizer is necessary in my question if you have these extra wings, and you would most likely have a t-tail stabilizer configuration on this aircraft if it did exist.

@PeterKämpf, it's been reopened. Though I believe there is a good explanation of longitudinal stability somewhere already and then this could be closed as duplicate.

Is horizontal stabilizer necessary for tandem wing aircraft? not if the main wings are capable of carrying out the functions of the tailplane.
The main reasons for horizontal stabilizer are twofold:

To provide stability for the aircraft, as the wing alone configuration is unstable.
To provide pitching moment as the long moment arm enables enough moment to be produced by small changes in the lift (via the elevators).

It provides other benefits too (like damping), but these are the main ones.
For aircraft having tandem wings (i.e. having two wings one behind the another), horizontal stabilizer may or may not be present depending on various factors.

If the two wings are able to provide stability and provide pitch moment by differential control of the control surfaces (elevators) on them, the tandem wing has no horizontal stabilizer, like the Scaled Composites Proteus.

"Scaled Composites Proteus in flight 1". Licensed under Public Domain via Commons.

In some cases, the above objectives may not be met with the wings alone and a horizontal tail may be required, like the Scaled Composites Advanced Technology Tactical Transport. Interestingly, the aircraft was flown in two configurations. The initial configuration had a cruciform tail.

Image Courtesy of Scaled Composites, LLC
According to DARPA report AD-A227- 498,

Analysis of the flight test results of the "Cruciform Tail" configuration disclosed a significant deficiency in the directional stability and engine-out minimum control speed of the airplane as well as some perceived loading difficulties due to low clearance of the aft cargo door.

As a result, the configuration was changed into a twin boom 'bronco' tail and flown.

Image Courtesy of Scaled Composites, LLC
There are some advantages with the tandem wing configuration, like:

For same weight, the tandem wings are more compact (i.e. have less wingspan) as both the wings produce lift (This was one of the reasons given by Burt Rutan). They have less frontal area and less parasite drag.
In case of tandem wings, both the wings produce lift, on either side of the center of gravity and produce moments about the c.g. in opposite directions. So, for stability, the c.g. is shifted forwards (for e.g. by having the power-plant forward). As the Angle of attack (aoa) increases, the lift force shift towards the rear and as the center of pressure shifts to the rear of c.g., the tail has to produce higher downward force to produce the required counteracting moment. As higher aoa is used mostly in landings, where more lift is required at lower speeds, this is not ideal. However, in case of tandem wings, at higher aoa, lift force of both the wings move to the rear, resulting in the rear wing having longer moment arm, and by extension, larger moment. So to compensate, the lift produced by the front wing has to be larger, which produces more lift, not less. For this reason, the front wing is set at a higher incidence (so as to produce higher lift) compared to the rear one.

Source: nurflugel.com

In general, the c.g. of a tandem wing aircraft is towards the main forward wing with the rear wing at a smaller incidence compared to the forward one. This means that the forward wing will have higher wing loading and stall first. However, as the rear wing is still producing lift, the nose drops and the front wing starts to produce lift again, giving the pilot some time to react and control the aircraft, unlike the conventional configuration.

As the wingspan is less, the induced drag is more though.


text:
Can the V-22 carryout vertical takeoffs with shorter blades?
The V-22 has blades that are larger than blades on most helicopters. Could you make them smaller and still have the vertical takeoff effect?
Bonus: Does the V-22 get some of its thrust from the exhaust?

The rotor size helicopter (or a tiltrotor, in this case) is determined by its aerodynamic requirements, most importantly, its hover characteristics. The main constraint is the weight that has to be lifted by the helicopter at a give a altitude.
In this context, it can be seen that the blades of V-22 are actually smaller. For example, V22 has a Max. TOW(Take-off Weight) of 27,400 kg. For achieving this, the V22 twin rotors of 11.6 m diameter, giving a rotor disc area of 212 m². A CH47, which has a Max TOW of 22,680 kg has a rotor disc area of 520 m² using twin rotors.
If we take single rotor helicopters, the CH 53 Super stallion has a 24 m dia. rotor having a disc area of  460 m² to lift a max TOW of 33,300 kg.
This lesser blade disc area of V22 limits is ability to hover at higher altitudes. The V22 rotor blades will have been optimized for required performance parameters. So, if the blade length is reduced (assuming airfoil is kept the same), vertical takeoff is possible though other performance characteristics will be severely affected.
The V 22 uses two Rolls-Royce Allison T406/AE 1107C-Liberty turboshafts, which do not produce any thrust.

Strictly speaking, all jet engines produce some thrust since they must vent hot expanding gases somewhere.  Whether that thrust is significant or useful is another matter.

You're correct in saying that all jet engines produce thrust. However, in case of turboshaft engines in general, the residual thrust is not significant. Even when some residual thrust is available, steps are taken (like turning the exhaust) to reduce their effect.  My answer was in that context.


text:
Which commuter flight was forced to land in a cornfield in '96 or '97?
Last night a friend's wife was describing her experience as a child being on a flight that encountered a flock of Canadian geese, where at least one engine was lost to a bird strike. I suspect both engines may have been lost, or at least compromised, because she said the plane ultimately landed in a cornfield. She said the oxygen masks deployed, and she used the inflatable slide to deplane.
If her memory is correct the flight departed Indianapolis, IN, bound for Trenton, NJ. Based on her age, we thought the flight was probably in 1996 or 1997. She said it was a "small plane", but it was certainly a commercial flight, and I believe a jet (not a turboprop). She described two seats straddling the aisle.
Her memory of the event emphasized surreal, sensory details, so I suspect some uncertainty among the facts useful for finding this flight.
TL;DR here are the useful details:

Probably 1996 or 1997
Departed Indianapolis, IN
(Update 2.20.2018: She confirms it was Indianapolis International)
Intended arrival in Trenton, NJ
Engine(s) out, presumably due to bird strikes (Canadian geese)
Emergency landing in a field (possibly a cornfield)
(Update 2.20.2018: This was a Pennsylvania cornfield)

To my surprise I've been unable to Google more information, so I'm hoping someone here has a better angle on researching this interesting flight.

Nothing jumps out from https://en.wikipedia.org/wiki/Bird_strike#History_of_Incidents

Are you sure the aircraft ever became airborne? I.e. did it land in a cornfield or could it have been an overrun after an aborted take-off?

No matching incidents here either: https://en.wikipedia.org/wiki/Indianapolis_International_Airport?wprov=sfla1 KIND isn't exactly surrounded by fields, so an incident in close proximity to the field seems unlikely.

And no matches records on the ASN database (Collisons with birds) https://aviation-safety.net/database/dblist.php?Event=COOB

Enter search terms on NTSB website https://ntsb.gov/_layouts/ntsb.aviation/index.aspx

Thanks for the helpful feedback. Some good questions here. If possible I'll try to get some more specific details.

The only crashes I found that were even remotely close are United Express 5925 (not her flight, all involved were killed) and the propeller failure of the ASA commuter in the Southeast (1995). Nothing that matches the incident and location you describe, though.

One seat on each side of the aisle would suggest a small turboprop plane like the Beech 1900. Even the small regional jets have a 1-2 layout. And a plane of such size would be low enough to not need to have inflatable slides.

It is unclear what you mean by two seats straddling the isle. Do you mean a pair of seats on either side of the isle (4 across) or do you mean two seats across? And for everyone else, note that this detail may not be helpful as an upper class cabin arrangement could feature only one seat on either side of the isle (2 across), and this could be the detail remembered.

It's really tough to hunt down non-fatal accidents without some specific info. The sheer volume of mishaps is hard to wade through. The NTSB site lists 2270 accidents just in 1996

What about this: CHI971A029.  https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20001208X07001&AKey=1&RType=Final&IType=IA

@wbeard52 thanks for staying on the hunt, but I doubt that's it. It might be best to put this question on ice until I can ask her for some more specific details. Honestly, I expected this to result in a quick answer from wiser av researchers like yourself. That it remains a mystery based on the details I was provided has me wondering if the flight as remembered  has radical variations from the actual flight. In other words, I'm now fearing it may have gained theatrical embellishments over the years. In hindsight, I probably wasn't expected to turn around and fact-check it haha :)

I am skeptical that there was ever scheduled non-stop service between Indianapolis and Trenton; IND, while sizeable, is not huge, and Trenton is a small airport that has intermittently gained and lost scheduled service over the years.  Eastwind Airlines did use it as a hub in the '90s, but as far as I can tell they never flew to Indianapolis.

Nothing on the Wikipedia lists for aviation incidents/accidents in either years refers to cornfields or anything of the like. There was in incident in the 70s, I believe, in which what looked like a DC3 belonging to the USA army crashed into a cornfield. It killed everyone on board, however. Doesn't sound like your story at all. Sorry pal

The only commuter aircraft off-field landing in a corn field that I can find that even comes close to the description is this one, an Aspen Airways/United Express Convair 580, but it was in January 1989, and in Colorado.
https://www.ntsb.gov/_layouts/ntsb.aviation/Results.aspx?queryId=0b239aa8-3596-401c-ae5c-e4322035f7aa
I found the story in the Los Angeles Times archive here: http://articles.latimes.com/keyword/commuter-plane

Aviation-Safety Network lists all major aircraft accidents and incidents. Here is a list sorted by "bird strike": https://aviation-safety.net/database/dblist.php?Event=COOB

@MarkJonesJr., if you see my comment above I've since assumed the details I was given are a combination of truths, misrememberings, and a few years worth of embellishments. So I'm not sure this question is answerable, as asked. However if you'll write an abstracted answer combining wbeard52's comment about ntsb.gov and your comment mentioning aviation-safety.net, with appropriate search terms and such, I'll give you the accepted answer. In lieu of being, answerable, I think the best answer possible is one that demonstrates how to research a flight like this in the first place. :)


text:
How did the Wright brothers measure the maximum airspeed of their 1904 plane?
The Wright brothers used during their 1904 trials (105 in total, according with their claims) a Richard anemometer mounted on Flyer II (there are numerous pictures on the net with such a device). The instrument just integrates the airspeed and when you stop it you read the displayed distance and divide it by the flight time and so get the average airspeed of the airplane during the entire flight.
The question is: How did the Wright brothers measure that maximum airspeed of 70 ft/s that occured during a flight characterized by a 59 ft/s average airspeed (see the letter to Octave Chanute below)?
Wilbur Wright, “Letter to Octave Chanute (the mentor of the Wright brothers)”, Dayton, August 28, 1904.

“Since the first of August we have made twenty five starts with the #2
Flyer. The longest flights were 1432 ft., 1304 ft, 1296, ft. and 1260
ft. These are about as long as we can readily make on our present
grounds without circling. We find that the greatest speed over the
ground is attained in the flights against the stronger breezes. We
find that our speed at starting is about 29 or 30 ft per second, the
last 60 ft of track being covered in from 2 to 2¼ seconds. The
acceleration toward the end being very little. When the wind averages
much below 10 ft per second it is very difficult to maintain flight,
because the variations of the wind are such as to reduce the relative
speed so low at times that the resistance becomes greater than the
thrust of the screws. Under such circumstances the best of management
will not insure a long flight, and at the best the speed accelerates
very slowly. In one flight of 39¼ seconds the average speed over the
ground was only 33 ft per second, a velocity only about 3 ft per
second greater than that at starting. The wind averaged 12 ft per
second. In a flight against a wind averaging 17 ft per second, the
average speed over the ground was 42 ft per second, an average
relative velocity of 59 ft per second, and an indicated maximum
velocity of 70 ft per second. We think the machine when in full flight
will maintain an average relative speed of at least 45 miles an hour.
This is rather more than we care for at present.  Our starting
apparatus is approaching completion and then we will be ready to start
in calms and practice circling.”

Richard anemometer

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Was Stringfellow's 1848-1868 motor a revolutionary aviation engine or just a useless low efficiency toy which never powered a flying model plane?
I took a look at the 1848 Stringfellow's aviation steam engine and I do not really understand how it was capable to power a model plane in horizontal flight as long as the motor itself was just an ordinary toy of the time and the boiler is a below-average design, as long as the fire is not surrounded by water from all sides to transfer the energy of the fuel with maximum efficiency to the working fluid. The fire burns around the upside-down cones containing water and it radiates a good part of its energy through the thin metal walls of the boiler.
The Scientific American (Vol. 20, Iss. 11, p.69; Mar 13, 1869) says that a later version of this engine (1868) achieved a pressure of 100 $lb/in^2$, after 7 minutes from the moment the fire was started, and was able to turn two propellers 3 ft in diameter at 300 rev/min. The dry mass of the motor and boiler together was 13 lb.

IS A FLYING MACHINE A MECHANICAL POSSIBILITY?
… The organization of the Aeronautical Society, which gave its first
exhibition at London last June, is an evidence that the belief is
gaining rather than losing ground. … what are we to say of an engine
weighing only sixteen lbs., and being able to work to one-horse power?
The council of the society voted their £100 prize to Mr. Stringfellow
for an engine of this description; and whether or not it ever becomes
the motive power for flight, it would seem, from its ingenuity, to be
well worth the reward. “The cylinder,” the report tells us, “is 2
inches in diameter, stroke 3 inches, and works with a boiler pressure
of 100 lbs. to the square inch; the engine working 300 revolutions per
minute. The time of getting up the steam was noted; in three minutes
after lighting the fire the pressure was 30 lbs.; in five minutes, 50
lbs.; and in seven minutes there was the full working pressure of 100
lbs. When started, the engine had a fair amount of duty to perform in
driving two four-bladed screw propellers, 3 feet in diameter, at 300
revolutions a minute.”
The data for calculating the power are taken as follows: Area of
piston, 3 inches; pressure in cylinder, 80 lbs. per square inch;
length of stroke, 3 inches; velocity of piston, 150 feet per minute; 3
× 80 × 150 = 36,000 foot-pounds. This makes rather more than one-horse
power (which is reckoned at 33,000 foot-pounds). The weight of the
engine and boiler was only 13 lbs., and it is probably the lightest
steam engine that has ever been constructed. The engine, boiler, car,
and propeller together were afterwards weighed, but without water and
fuel, and were found to be 16 lbs.”
This engine seems to demonstrate the possibility of making engines
light and powerful enough for purposes of flight.

However, nothing is said about the capacity of the boiler to maintain a suitable pressure for, say, at least 30 seconds, an interval of time that would have allowed a short flight. Another thing would be, if this engine was able to fly a model plane in 1848, why did it not repeat the performance in 1868?

Source: John Stringfellow, engine, 1848
With the data in the Sci. Am. article of Mar 13, 1869, I calculated that Stringfellow's engine needed 1.54 liters of steam at 5.44 atm each second in order to work at the desired power.
Judging from the total size of the recipients that contained the steam (which is below 15 liters) and the time needed to reach the necessary pressure, of the order of 7 minutes, this engine would have functioned at most 10 seconds and then it would have needed a few minutes to build up again enough pressure for another few seconds of generating mechanical power.

Source: The Conquest Of The Air
Question: Was John Stringfellow's 1848-1868 motor a revolutionary aviation engine or just a low efficiency toy well behind its time? Was this device really capable to power the 1848 model aeroplane of the above mentioned inventor?
(A link to an exact working replica of the 1848 engine and plane would answer my question immediately.)

I am not sure at all Stringfellow's engine ever powered a model plane that really flew. The story of the 1848 flights is written in a pamphlet "A few Remarks on what has been done with screw-propelled Aeroplane Machines from 1809 to 1892" written by F. J. Stringfellow, the son of the inventor (see: https://www.ctie.monash.edu/hargrave/stringfellow.html). Basically, Stringfellow Jr., declared that his father was the first person who built a working heavier-than-air flying machine. This is just a claim. (In 1892 there were model planes capable of flight, built by others.)

Agree with Dracula, to suit this site, the question should be about flight, not about the viability of the engine. An interesting question even as it originally stands, but as such maybe better suited for engineering stack exchange or some other site on SE.

I have already updated the question adding "Was this device really capable to power the 1848 model aeroplane of the above mentioned inventor?"

There is absolutely no evidence that somebody before Alphonse Penaud flew model planes. This inventor flew his Planophore at Tuileries Gardens in Paris, France, on August 18, 1871. The Planophore, powered by a rubber band driven pusher propeller, flew 181 feet in 11 seconds. (see: http://www.flyingmachines.org/pend.html). John Stringfellow himself never claimed powered flights.

Had the 13 lb engine+boiler of John Stringfellow really generated 1 hp, continuous power, it would have become, then in 1848, even 1868, an extremely successful motor for powering tools (like a lathe) in small workshops, for powering small river boats, steam cars, draisines, even bicycles. In reality, Stringfellow's motor was a nothing. It worked just in pulses of a few seconds, followed by minutes of pause necessary for rebuilding the steam pressure.


text:
Is there strong evidence that Clement Ader flew a plane on Oct. 9, 1890 or at any date before 1897?
About Clement Ader it is often said that on October 9, 1890, he flew a man carrying plane for a distance of about 50 meters.
After searching the archives the best evidence I found, in support of this assertion, is an article published on July 11, 1891, and signed Gaston Tissandier (French chemist, aeronaut and founder of La Nature). The author of the text says that after reading stories, in newspapers, about a machine built and flown by C. Ader, the inventor was interviewed and he proved to be quite secretive but finally, assaulted by insistent questions, Ader said his plane had risen to 50 cm to 1 meter, above the ground. The date of the trial is not specified and G. Tissandier does not appear to be convinced that Ader told the truth.
Question: Are there other articles or documents of the time that support the claim that on October 9,  1890 or at any date before 1897, Clement Ader flew a heavier-than air machine?
(As a remark: This is not a question about the experiments of Ader with his 1897 plane, financed by the French government.)
This is an approximate English translation (made by me) of the French article:
"Mr. Ader's mechanical bird. - About a fortnight ago, several of our most popular political journals announced that Mr. Ader, the scientist and pleasant electrical engineer, had built a marvelous aircraft which would bring to the field of science the much sought-after discovery of air navigation. It has been asserted that on a property at the outskirts of Paris, Mr. Ader rose with a mechanical bird having a powerful propulsor; and since then, the news has continued to travel in the world, somewhat amplified by the echoes. We wanted to know what was true in these stories, and we went to see Mr. Clément Ader, whose telephone and endless rails we once described.
To our regret, it was not possible for us to collect the precise information we would have liked. Mr. Ader told us that the description of the experiment, as presented by the newspapers, was grossly exaggerated, but the fact of building an aircraft was true. The ingenious physicist claims to have found a practical aviation device; he refuses to give a description of the engine that drives it, and he has no intention of subjecting it to further tests, for the time being. Mr. Clément Ader, as a result of our insistent questions, told us that he had actually carried out a decisive experiment. His mechanical bird, built after ten years of experiments and work, is shaped like a large bat with outstretched wings. The apparatus is constructed of hollow wood and silk; when the wings are opened, they span 16 meters. The animal's body is hollow and houses the aviator and the engine. This very light and very powerful motor operates a propeller which make the aircraft move. The wings do not flap and remain opened. When the propeller begins to work, the mechanical bird, mounted on wheels or skates, runs on level ground, with a fairly high speed, then little by little it finds its fulcrum in the air, and it manages to rise from the ground and continue its ascent. This is what Mr. Ader told us. - But, we asked, how high could the device rise? - Here Mr. Ader does not give precise information; as we insisted however, the inventor told us that he had risen to a low height, and, to give us an idea of the elevation, he rose his hand at 50 centimetres or 1 meter above the ground. - Does the device operate for a significant period of time? - M. Ader replied that the time was short, but that he intended to build an apparatus which would rise higher and would function for a longer period. The inventor intends to mount his mechanical bird and show it, not to the public, but to scientists and friends; he added that he would show it in an inert state, that he had no plans to make it work, that he had spent considerable sums on his studies and that he needed capital to continue and develop its constructions.
Mr. Ader is too good a physicist not to know that in the field of experimental science, demonstrations must be made by experiment; both scientists and capitalists will all ask him for it, and whatever good opinion we may have of the inventor's merit, it is only after the experiment, when it is presented to us, that we draw our conclusions.
G. T."
The French original
Gaston Tissandier, "L'oiseau mécanique de M. Ader.", Supplément à « La Nature » du 11 juillet 1891 (n° 945), Publié sous la direction de M. Gaston Tissandier, p. 6.

L'oiseau mécanique de M. Ader. — Il y a une quinzaine de jours,
plusieurs de nos journaux politiques les plus répandus ont annoncé que
M. Ader, le savant et sympathique ingénieur-électricien, avait
construit un merveilleux appareil d'aviation qui allait apporter dans
le domaine de la science la découverte tant cherchée de la navigation
aérienne. On a affirmé que l'on avait vu, dans une propriété des
environs de Paris, M. Ader s'élever dans un oiseau mécanique, muni
d'un puissant propulseur; et depuis, la nouvelle continue de courir le
monde, quelque peu amplifiée par les échos. Nous avons voulu savoir ce
qu'il y avait de vrai dans ces récits, et nous avons été voir M.
Clément Ader, dont nous avons jadis décrit le téléphone et les rails
sans fin.
A notre regret, il ne nous a pas été possible de recueillir
les renseignements précis que nous aurions désiré. M. Ader nous a
affirmé que le récit de l'expérience annoncée par les journaux était
fort exagéré, mais que le fait de la construction d’une appareil
d'aviation était vrai. L'ingénieux physicien affirme avoir trouvé un
appareil d'aviation pratique; il se refuse à donner la description du
moteur qui l'anime, et il n'a pas l'intention de le soumettre à de
nouveaux essais, quant à présent. M. Clément Ader, sur nos
interrogations pressantes, nous a dit qu’il avait fait, en réalité une
expérience décisive. Son oiseau mécanique, construit à la suite de dix
années d'expériences et de travaux, a la forme d'une grande
chauve-souris aux ailes étendues. L'appareil est construit en bois
creux et en soie; quand les ailes sont étendues, elles ont 16 mètres
d'envergure. Le corps de l’animal est creux et abrite l'aviateur avec
le moteur. Ce moteur, très léger et très puissant, fait agir une
hélice qui imprime le mouvement à l’appareil. Les ailes ne battent pas
et restent étendues. Quand l’hélice commence à fonctionner, l'oiseau
mécanique, monté sur des roues ou des patins, court sur un terrain
uni, avec une assez grande vitesse, puis peu à peu il trouve son point
d'appui dans l’air, et il arrive à se soulever du sol et à continuer
son ascension. Voilà ce que nous a raconté M. Ader. — Mais, avons-nous
demandé, à quelle hauteur l'appareil peut-il s'élever? — Ici M. Ader
ne donne pas de renseignements précis; comme nous insistions
cependant, l'inventeur nous a dit qu'il s'élevait à une faible
hauteur, et, pour nous donner une idée de l'élévation, il étendait sa
main à 50 centimètres ou 1 mètre au-dessus du sol. — L'appareil
fonctionne-t-il pendant un temps d’une durée appréciable? — M. Ader
nous a répondu que ce temps était court, mais qu'il se proposait de
confectionner un appareil qui s'élèverait plus haut et fonctionnerait
pendant une durée plus considérable. L'inventeur se propose de monter
son oiseau mécanique et de le faire voir, non pas au public, mais à
des hommes de science et à des amis; il a ajouté qu'il le montrerait à
l'état inerte, qu’il n'avait pas le projet de le faire fonctionner,
qu'il avait dépensé des sommes considérables pour ses études et qu'il
lui fallait des capitaux pour continuer et développer ses
constructions.
M. Ader est trop bon physicien pour ne pas savoir que,
dans le domaine de la science expérimentale, les démonstrations
doivent se faire par l’expérience; les hommes de science comme les
capitalistes la lui demanderont tous, et quelque bonne opinion que
nous puissions avoir du mérite de l'inventeur, c’est de l'expérience
seule, quand il nous sera donné de la voir, que nous tirerons notre
jugement.  G. T.

This is not an answer as I have no definitive information, but in general when an inventor refuses to demonstrate or give evidence on progress it means there's nothing to show.

The behavior of Clement Ader's 1897 plane represents strong evidence against any flight performed by the same inventor in 1890 and 1891 using similar machines.
The Oct. 21, 1897 report, made by general Mensier, and its annex of Oct. 27 do not leave room for interpretation. The 1897 airplane never left the ground with its front wheels during the Oct. 12 and 14 officially witnessed trials, but the rear wheel rose into the air.
Ader's Avion III is a fundamentally flawed design. A model tested in 1990 (see the video below) behaved identically. The rear wheel leaves the runway but finally the apparatus leans forward too much and hits the ground with its propellers. The model, like the original, also has the tendency to veer to one side. In the case of the 1897 plane, once the rear wheel no longer touched the runway, the only means that remained for steering the plane was the vertical rudder which (probably) was inefficient at low speeds.
Had C. Ader flown in 1890 and 1891 he would have built in 1897 a machine at least as good as the previous two apparatuses.

Model of Ader 1897 plane, tested in 1990, which behaved multiple times in a similar way as the real airplane.

Just because I'm upvoting, doesn't mean I believe that this is the absolutely correct answer, but there is some useful research effort here--

It is generally accepted by aviation historians that during a test at 9 October 1890 the wheels of Ader's machine briefly left the ground just barely and came back down 50 meters away. Later claims which Ader made in a book Military Aviation (Ader, Clément, "La première étape de l'aviation militaire en France", Paris, J. Bosc et Cie, 1907) published in 1909 are treated with more skepticism.
His claim of a 50-meter "powered hop" is plausible based on the size and weight of his machine and the engine power on board. He made the claim immediately after the event and there was at least one witness. On this basis he is given credit for building the first heavier than air machine which was able to rise from the ground unassisted. He has not received and did not claim credit for building a working airplane.
According to Sobolyev (История самолетов мира, 2001, p. 31) there is a document prepared by one of Ader's assistants in which he states:

Avion number 1, bearing the name "Eole" and under the control of Mr. Ader its inventor broke away from the earth and maintained itself in the air on its wings shaving the surface of the earth for a distance of approximately 50 meters with the help of but a single source: its own thrusting power. The test surface had been tamped with a roller and had a length of 200 meters and a width of 25 meters.

Sobolyev also cites a personal letter from Ader to Nadar dated 12 October 1890 (three days after the experiment). In it he says:

I have finally solved the problem after wearying labor and considerable monetary expense. My finished machine is called "Eole"; it has just now performed its first takeoff with me on board over a distance of 50 meters; this distance could not have been greater because the test surface was too small.

Both quotes above are my translations of Sobolyev's translations into Russian. I do not know whether Sobolyev translated Ader's from an English translation or directly from the French.
Sobolyev does not claim to have seen either document. For the statement by the assistant his citation is "Dollfus C., Bauche H. Histoire de l'aernoautics. Paris, 1932, P. 140". For the letter to Nadar his citation is "Gibbs-Smith C. Clement Ader.His flight-claims and his place in history. London, 1968. P. 9". (Citations copied verbatim.) Sobolyev says that the letter is in the National Library in Paris.
The claims which Ader made in 1917 are only slightly greater than what he made to Nadar in 1890. He claims to have made more hops which went a bit further and one big hop when a strong gust of wind hit the machine while it was in the air. If there is enough interest, I can add details later.

In an answer, I will translate for you and others,  the first part of the work: "Ader, Clément, "La première étape de l'aviation militaire en France", Paris, J. Bosc et Cie, 1907". In his book Ader does not support with documents his claimed flights of 1890 and 1891. He bluntly declared himself the father of aviation.

@duofilm There is a 2003 English translation of Ader's book from Air University Press. The US Department of Defense has a scan of it on one of their websites. https://media.defense.gov/2018/Jan/04/2001862907/-1/-1/0/B_0011_ADER_MILITARY_AVIATION.PDF

The translation you mentioned is of another book by Ader as long as the inventor says at one point, see page 10, : "Now, writing these lines in October 1908". The book, that I translated in part, was published in 1907.

The ancestor of airplanes was Éole. ("L’Aïeul des Avions fut l’Éole." - Clement Ader)
In December 1906, when it was already demonstrated in front of official witnesses, that the man carrying planes can take off and fly, Clement Ader wrote a book, published in 1907, in which he established himself as the father of aviation. He claims flights for all his three planes of 1890, 1891 and 1897 but for the first two machines the inventor does not bring forward any evidence (old letters, articles, declarations of witnesses, etc.) in support of his pretentions.
Here is an electronic translation (brushed a bit by me) of the first part of Ader's 1907 book:

The first stage of the military aviation in France
Paris, December 1906.
The text that follows does not pretend to be a history of Aviation.
Others, better documented, will write it one day and it will not fail
to be quite interesting. We will present here only a particular fact
belonging to the Military Aviation, and we will report it exactly as
it happened.
The name Avion [airplane], used to designate the flying machines
intended for War, comes, like that of Aviation, from the Latin
word Avis which means bird.
The ancestor of Airplanes was Éole. Being dominated by this firmly
established idea, that aviation should be used above all for National
Defense, we always conducted our work, since 1882, in the greatest
secrecy. Following extensive research on the flight of birds, on
Aerodynamics and special mechanics for Air Navigation, Éole was built
in a laboratory located on Pajou street in Passy. It consisted of two
wings similar to those of bats; they could be folded. The motive power
was provided by steam and it turned a propeller placed in front of the
plane. Two non-steerable wheels carried the aircraft and a third, at
the rear, steered it on the ground. A rudder was used for steering the
apparatus in the air. After being completed, it was necessary to
choose, for trying the plane, a discreet and quiet place. We found it
in the Parc d’Armainvilliers, belonging to Mrs. Isaac Pereire, and it
was made available to us by her son, Mr. Gustave Pereire.
A straight line runway was built, the grass was removed, the soil
compacted and leveled by a roller, so as to make visible and record
the traces left by the wheels, from the smallest decrease in the
pressure exerted by the wheels on the ground to complete lift-offs.
During one of these experiments, on October 9, 1890, over a distance
of about 50 meters, Éole lost contact with the ground for the first
time, using only its motive force. This small event was not recorded
in a written report, but our foremen buried blocks of coal in the
ground at the exact spot where we took-off in our aircraft. What is
certain is that these witnesses are still on the surface of the ground
[alive] and that it would be possible to find them if this thing
became necessary. The experiments did not continue due to a major
damage the steam generator suffered.
At the time, Eole was considered, by informed people and by the
Scientific Press, as the first aerial device, mounted by a man, having
flown at a low height and over a short distance.
The following year, in 1891, the generator of Eole was rebuilt and
improved. The experiments were repeated in September, on a larger
field, with the authorization of the Minister of War, at Camp de
Satory. The area [the runway] was still straight like the first, but
much longer, about 800 meters, - previously leveled and compacted.
Longitudinally, in the middle of the runway, there was a white stripe,
1-meter wide, for guiding the aviator who was still ourselves. Several
tests were made, flying here and there [hoping] and during one of
these experiments Éole left the runway to the left for a length of
about 100 meters, without leaving any marks on the ground. It remained
at a low height, but completely supported by its wings. This deviation
to the left, while flying, was caused by the central single propulsor
as soon as the plane left the ground, an effect which we will explain
later. Unfortunately, the machine came up violently against an ensemble of
carts, buckets, etc. ... which had been used to trace, with lime, the
central line. The apparatus was damaged to the point of not being able
to continue the tests. The strong shaking we felt made us think about
the drawbacks of a single central traction.
However, the results of these tests spread discreetly inside a small
group of officers, and so we had the honor to be visited by Major
General Mensier, then Director of the 4th Directorate (engineering) at
the Ministry of War. Due to his kindness we had already owed the
ministerial authorization which had enabled us to operate on a
military terrain. We could only show him an Éole damaged and
immobilized in its shelter.
A few days later, the damage to the aircraft was provisionally
repaired; Mr. Berger, former Commissioner General of the Universal
Exhibition, currently deputy of the Seine, who was very interested in
our invention, had obtained for us from the Prefect of Seine the
authorization to exhibit the machine in the Pavilion of the City of
Paris, located at that time next to the Palais de l’Industrie. With
some friends we had planned to present it to the world of scientists,
of Finance and to the Press, having the intention of establishing a
Research Society for continuing the aviation experiments. The airplane
was about to be shown when M. de Freycinet, who was then President of
the Council and Minister of War, expressed his desire to see it. He
came, on Saturday, October 17, 1891, accompanied by General Mensier.
Once his examination was completed, he took a decision: “This, he
said, is an “Aerial Scout and Torpedo Craft” and the tests must be
continued by the War Department for the National Defense. This is how
the Military Aviation was born that day through the will of M. de
Freycinet. This made us happy as long as we held the same beliefs.
We immediately made a general plan, and we were justified to do so,
this was not a bold decision, by the numerous preliminary studies
carried out from 1882 to 1891, as well as by the results obtained with
Éole. Here are the main points: - Foundation of a school for Aviation
and an Aviation Industry. - Establishment of a Military Workshop for
building airplanes. - Air strategy and tactics. - Creation of an Air
Force.
To fulfill this program, no doubt, resources were needed proportional
to its importance; but could these expenses and the great projected
benefits for the National Defense be really put in balance?
So, trusting the words of M. de Freycinet and in support of General
Mensier, we started the work, without even waiting for written
agreements establishing the conditions under which the work had to be
carried out. This thing we did later.
The old laboratory-workshop on Pajou street which produced Éole became
insufficient. It was necessary to build another, larger and more
complete. This construction work took place in 1892, Jasmin street, 11
and 13, in Auteuil. It consisted of a mechanical workshop; another for
modeling, the frames and canvas; several other rooms for the forge,
stores, design workshop, office, study; a high roof room, 20 meters
long by 10 wide, for tests, plots and assembling aircraft frames. In
the courtyard and other related places, there were several more small
buildings, sheds and the guard’s hut. The motive power was provided by
a steam engine which powered many tools. The heating was done by
heaters and by steam. We had to build a true factory, and this was
necessary because of the complex activity that we had to carry out.
The establishment was placed under the rigorous jurisdiction of
military laws and became, in fact, the first school of the Aviation
Industry. An uninterrupted work was conducted in the most absolute
secrecy. There, many inventions concerning Aviation, and a lot of
manufacturing processes, were born, known only to our staff, and which
constituted the nascent Aircraft Industry. ...

The book continues with the activity of Ader related to the construction and testing of his 1897 plane which, as I understand, is not a subject of interest for the current question.
The original text in French as written by Clement Ader in December 1906:
1907, Ader, Clément, "La première étape de l'aviation militaire en France", Paris, J. Bosc et Cie.

La première étape de l’aviation militaire en France
Paris, Décembre 1906.
Ce qui va suivre n’a pas la prétention d’être une histoire de
l’Aviation, d’autres que nous, mieux documentés, l’écriront un jour et
elle ne manquera pas d’être très intéressante ; il ne s’agira ici que
d’un fait particulier qui appartient à l’Aviation Militaire, et nous
allons le raconter fidèlement tel qu’il s’est accompli.
La dénomination d’Avion pour désigner les appareils volants destinés à
la Guerre, dérive, ainsi que celle d’Aviation, du mot latin Avis qui
signifie oiseau.
L’Aïeul des Avions fut l’Éole. Étant pénétré de cette idée, bien
arrêtée, que l’aviation devait surtout servir à la Défense Nationale,
nous avions toujours conduit nos travaux, depuis 1882, dans le plus
grand secret. A la suite de longues recherches sur le vol des oiseaux,
sur l’Aérodynamique et la mécanique spéciale à la Navigation Aérienne,
l’Éole fut construit dans un laboratoire situé rue Pajou à Passy. Il
se composait de deux ailes semblables à celles des chauves-souris ;
elles se repliaient. La force motrice était fournie par la vapeur et
faisait mouvoir une hélice placée à l’avant. Deux roues droites
portaient l’appareil et une troisième à l’arrière le dirigeait à
terre. Un gouvernail servait à la direction aérienne. Après son
achèvement, il fallut choisir, pour l’essayer, un endroit discret et
tranquille ; nous le trouvâmes dans le Parc d’Armainvilliers,
appartenant à Mme Isaac Pereire et qui fut mis à notre disposition par
son fils, M. Gustave Pereire.
Une aire fut tracée en ligne droite, le terrain dégazonné, battu et
égalisé par un rouleau, de manière à voir et à enregistrer les traces
des roues depuis les plus petits allègements jusqu’aux soulèvements
complets. A l’une de ces expériences, le 9 Octobre 1890, sur une
distance d’environ 50 mètres, l’Éole perdit terre pour la première
fois, avec la seule ressource de sa force motrice. Ce petit événement
ne fut constaté par aucun procès-verbal, mais nos contremaîtres
enfouirent en terre des blocs de charbon à l’endroit précis où nous
nous étions élevé dans notre appareil. Ce qu’il y a de certain, c’est
que ces témoins sont encore dans le sol et qu’il serait possible de
les y retrouver si cela devenait nécessaire. Les expériences ne
continuèrent pas par suite d’une grosse avarie survenue au générateur
de vapeur.
A cette époque, l’Eole fut considéré, par les initiés et par la Presse
scientifique, comme le premier appareil aérien monté par un homme,
ayant volé à une faible hauteur et sur une petite distance.
L’année après, en 1891, le générateur de l’Éole était refait et
perfectionné, les expériences furent reprises au mois de Septembre,
sur un terrain plus vaste, avec l’autorisation du Ministre de la
Guerre, au Camp de Satory. L’aire était encore droite comme la
première, mais bien plus longue, 800 mètres environ, - préalablement
nivelée et battue. Il y avait longitudinalement, au milieu, une raie
blanche de 1 mètre de largeur pour orienter l’aviateur qui était
encore nous-mêmes. Plusieurs essais furent faits en voletant et à
l’une de ces expériences L’Éole sortit de la piste à gauche sur une
longueur d’environ 100 mètres, sans laisser de traces sur le sol, il
était à une faible hauteur, mais complètement soutenu par ses ailes.
Cette déviation à gauche, en volant, fut causée par le propulseur
unique central dès que l’Avion fut libre en perdant terre, effet que
nous expliquerons plus tard. Malheureusement, il vint buter violemment
contre un matériel de chariots, de baquets, etc... qui avaient servi
tracer, à la chaux, la raie centrale ; l’appareil s’avaria au point de
ne plus pouvoir continuer les essais. La forte secousse que nous
ressentîmes nous fit réfléchir sur les inconvénients d’une traction
unique centrale.
Cependant, les résultats de ces essais s’ébruitèrent discrètement dans
un petit cercle d’officiers, ce qui nous procura l’honneur de la
visite de M. le Général de Division Mensier, alors Directeur de la 4°
Direction (génie) au Ministère de la Guerre, à l’obligeance duquel
nous devions déjà l’autorisation ministérielle qui nous avait permis
d’opérer en terrain militaire. Nous ne pûmes que lui montrer L’Éole
avarié et immobilisé dans sa baraque-abri.
Quelques jours après, les avaries de l’appareil étaient provisoirement
réparées ; M. Berger, ancien Commissaire Général de l’Exposition
Universelle, actuellement député de la Seine, qui s’intéressait
beaucoup à notre invention, avait obtenu pour nous du Préfet de la
Seine l’autorisation de l’exposer dans le Pavillon de la Ville de
Paris, situé à cette époque à coté du Palais de l’Industrie ; avec
quelques amis nous avions fait le projet de le présenter au monde
savant, de la Presse et de la Finance, en vue de la formation d’une
Société d’Études pour la continuation des expériences d’aviation. On
allait montrer l’Avion, lorsque M. de Freycinet, qui était alors
Président du Conseil et Ministre de la Guerre, témoigna le désir de le
voir le premier. Il vint, en effet, le samedi 17 Octobre 1891,
accompagné du Général Mensier. Son examen terminé, sa résolution fut
bientôt prise : « Ceci, dit-il, est un « Éclaireur » et un «
Torpilleur» Aérien et il faut faire continuer les essais par le
Département de la Guerre pour la Défense Nationale. » C’est ainsi que
naquit ce jour-là l’Aviation Militaire par la volonté de M. de
Freycinet. Cela nous combla de joie puisque nous-mêmes étions animé
des mêmes sentiments.
Nous fîmes immédiatement un projet général, et nous y étions autorisé,
sans témérité aucune, par les nombreuses études préliminaires faites
de 1882 à 1891, ainsi que par les résultats obtenus avec l’Éole. En
voici les branches principales : — Fondation d’une école d’Aviation et
d’Avionnerie. — Établissement d’un Arsenal pour la construction des
Avions. — Stratégie et tactique Aérienne. — Création d’une armée
Aérienne Aviatrice.
Pour satisfaire ce programme, sans doute, il fallait des ressources en
rapport avec son importance ; mais que pouvaient être ces sacrifices
en argent à côté des services qu’il était appelé à rendre à la Défense
Nationale ?
Donc, confiant dans la parole de M. de Freycinet et dans l’appui de M.
le Général Mensier, nous nous mîmes résolument à l’oeuvre, sans même
attendre d’avoir arrêté par des conventions écrites, sous quel régime
devaient s’exécuter les travaux ; ce que nous fîmes, d’ailleurs,
quelque temps après.
L’ancien laboratoire-atelier de la rue Pajou qui produisit l’Éole,
devenait insuffisant, il fallut en construire un autre plus vaste et
plus complet ; cela eut lieu en 1892, rue Jasmin, 11 et 13, à Auteuil
; il se composa : d’un atelier pour la mécanique ; d’un autre pour le
modelage, les charpentes et la voilure ; de plusieurs autres pièces
pour la forge, les magasins, atelier de dessin, bureau, cabinet de
travail ; d’une salle haute de toiture, de 20 mètres de long sur 10 de
large, pour essais, tracés et montage des charpentes des Avions. Dans
la cour et les dépendances il y avait encore plusieurs petits
bâtiments, des hangars et la loge du concierge. La force motrice était
fournie par une machine à vapeur qui faisait mouvoir de nombreux
outils. On obtenait le chauffage par des calorifères et par la vapeur.
Nous avions été amené à construire une véritable usine, et il le
fallait bien à cause des multiples travaux que nous avions à exécuter.
L’établissement fut placé, rigoureusement, sous la juridiction des
lois militaires et devint, de fait, la première École d’Avionnerie. Ce
fut alors un travail ininterrompu dans le secret le plus absolu : là
virent le jour quantité d’inventions concernant l’Aviation et une
foule de procédés de fabrication, connus seulement par notre personnel
et qui constituaient l’industrie naissante de l’Avionnerie. …